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Bearing Selection - How To Select A Bearing?
How to choose a bearing? - Explanation of The Bearings
And Guidelines And Tips For Selecting Bearings
Background
By Solaris2006 [ GFDL or CC-BY-SA-3.0 ], via Wikimedia Commons
Bearing is a mechanical component consists of two bodies when one is fixed and the other can
move relatively to the other. Bearing are very common in every motion mechanism, especially in
robotics and automation. The bearing allows motion between mechanical elements and lowers
the friction between them. The motion can be either angular or linear.
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Bearing Selection - How To Select A Bearing?
The first bearings were invented in ancient times. Large stones were placed on round wood logs
so they can roll on the logs. In this way the wooden logs were rotating between the stone and
the ground. Even after thousands of years, the principle I the same – only the size, the accuracy
and the technology have changed.
General Bearing Structure
Bearing internal structure consists of outer ring, inner ring, balls or other round shaped bodies,
cage and protective plates.
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Bearing Selection - How To Select A Bearing?
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How To Select A Bearing? - Bearing Specifications
Radial Force
Force acting perpendicular to the rotation axis of the bearing
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Bearing Selection - How To Select A Bearing?
Axial Force
Force acting Parallel to the rotation axis of the bearing
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Bearing Selection - How To Select A Bearing?
Combined Forces
A Combination of radial and axial forces:
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Bearing Selection - How To Select A Bearing?
Bearing Material
Material from which the bearing is made. There are many common materials such as: metal,
plastic and glass.
Rotation Speed
Every bearing has a maximum allowable rotation speed. As with any design of a mechanical
system, you should never design the bearing to rotate at 100% of the maximum allowable
speed. Safety factor should always be considered.
Shielded Bearing
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Bearing Selection - How To Select A Bearing?
Bearing protective plates that cover the entire structure of the bearing. Typically, protective
plates shall be fixed to the outer ring by “C” Shaped tightening ring. These plates provide good
protection against dust and dirt, but not perfect insulation. The protective plates are pressed
against the outer ring but there is always some very small gap in which dirt and dust can enter.
Sealed Bearing
Sealed bearing are protected bearing based on the same principle as shielding plates but in this
type of bearing the plates replaced with rubber plates. The rubber plates create perfect
insulation that prevent dust, dirt and even liquid to enter inside the bearing. However there is a
downside – the rubber ring may touch the inner ring and reduce the rotation speed of the
bearing.
Loading Direction
Loading direction is the direction of the stresses in the bearing. For example, the following
picture: 7 / 37
Bearing Selection - How To Select A Bearing?
© SKF
The bearing in the image placed in a way to withstand stress that exerted from the left side. If
the same force will be exerted from the right side, the bearing might fail. Therefore, it is very
important to analyze what is the load direction.
When using bearing with shifted rings (as in the picture), various loading direction can be
addressed by using several bearing arranged in three main methods:
Tandem Arrangement
© SKF
In this arrangement, the load lines are parallel and the bearing is distributing evenly the axial
and radial forces. However, this arrangement allows axial forces in one direction only. If
additional force is exerted on the opposite direction, or combined forces are expected, the a
third bearing or different arrangements should be considered.
Back-to-Back Arrangement
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Bearing Selection - How To Select A Bearing?
© SKF
In
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one bearing.
Axial forces
This
arrangement
Face-to-Face Arrangement
© SKF
In this arrangement, the load lines are acting towards the inside of the bearings. Axial forces
acting on both directions are handled well but each direction is handled with one bearing. This
arrangement is a less rigid then the last one and deals less with torque exerted on the bearings.
Basic Rating Life
Bearing is one of the few mechanical components that have an actual formula to calculate the
life time. The formula calculates the bearing life time according to standard ISO 281:1990:
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Bearing Selection - How To Select A Bearing?
When:
– Bearing life (in million hours)
– Characteristic of the bearing dynamic load (kN)
– dynamic load (radial + axial) on the bearing (kN)
– Lifetime Coefficient. P=3 for ball bearing and p=10/3 for sleeve bearing.
If the rotational speed is known and constant, then the following formula should be used:
When:
– Rotational Speed of the bearing (rpm)
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Bearing Selection - How To Select A Bearing?
–you
Life
time
(in
million
hours)
appropriate
with:
There
are
three
thumb
rules
about
the
life
time
of bearings.
bearing
lifeand
time
achived
1.
rotational
Lower
bearing
loads
3.
Larger
bearing
Diameter.
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time
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=speeds
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life
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time
itApplication
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based
aappliances,
recommended
bearing
the
should
application
toHigher
have,
follow
the
of
this
the
following
table
table
To
select
save
can
the
be
cost
L10h
Type
of
300
-12000
3000
Machines
used
inon
domestic,
agricultural
equipment,
instruments,
technica
3000
8000
intended
for
short
use:
Manual
freight
elevators
inbearing.
garages
and
workshops,
const
8000
Machines
designed
to
work
reliably
during
a
short
actions:
lifts,
cranes
10000
25000
but
not
regularly:
transmission
20000
30000
eight
hours
a
day
and
regularly:
metalworking
m
40000
50000
Machines
for
continuous
24
hours
a
day:
Turn
in
machining
equipment,
med
60000
100000
Water
industry
machinery,
cable
manufacturing
machines,
equipment,
vehic
>100000
electric machines, power plants, pumps Mines, Mine Fans
Large
Static Bearing Load
Static
load characteristic
a characteristic
of the
and indicated
the symbol and measured
in units ofisforce
(N). It is used
for bearing
calculations
related to by
bearings
rotating at
very low speeds (n <10rpm), and perform very slow periodic movements.
Dynamic Bearing Load
Dynamic
load characteristic
is a(N).
characteristic
of the
bearing and
indicated
the symbol and measured
in units of force
It is used for
calculations
related
to the by
rotating
bearings
and loads exerted on them.
Equivalent Static Bearing Load
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Bearing Selection - How To Select A Bearing?
© SKF
Equivalent static load is a characteristic that summarizes the forces acting on the bearing at rest
or at very low rotation speed. This is one of the most important characteristic when selecting
bearings. Equivalent static load is calculated using the following formula:
When:
– Equivalent Static Bearing Load (kN)
– Radial force exerted on the bearing (kN)
– Axial force exerted on the bearing (kN)
– Radial bearing load coefficient (each bearing has its own coefficient)
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Bearing Selection - How To Select A Bearing?
– Axial bearing load coefficient (each bearing has its own coefficient)
Static Safety Factor
Static Safety Factor is one of the characteristic that affect the decision whether to choose one
bearing or the other. The static safety factor is calculated using the following formula:
– Static Safety Factor
– Static Bearing Load Coefficient
– Equivalent Static Bearing Load
Checking against the following table for the case where the bearing is supposed to work: Type of Motion
Smooth movement without
Standard
vibration
Shocks
Ball Bearing (noise irrelevant)
0.5
0.5
>1.5
SleeveBearing (noise irrelevant)
1
1
>2.5
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Bearing Selection - How To Select A Bearing?
Ball Bearing (Standard)
1
1
SleeveBearing (Standard)1.5
>1.5
1.5
>3
Ball Bearing (Quite Motion)
2
2
>2
Sleeve Bearing (Quite Motion)
3
3.5
>4
Ball Bearing (Non-rotating)
0.4
0.5
>1
Sleeve Bearing (Non-rotating)
0.8
>2
* A Basic Table describing two bearings. Consult bearing manufacturers regarding these
figures.
If
calculated
static
factor a “stronger” bearing should be selected that fits the load
isthe
lower
than the
one safety
in the table,
requirements and the desired safety factors.
Equivalent Dynamic Bearing Load
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Bearing Selection - How To Select A Bearing?
© SKF
Equivalent Dynamic load is a characteristic that summarizes the forces acting on the bearing at
rest or at very low rotation speed. This is one of the most important characteristic when
selecting bearings. Equivalent static load is calculated using the following formula:
When:
– Equivalent dynamic load (kN)
– Radial force on the bearing (kN)
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Bearing Selection - How To Select A Bearing?
– Axial force
on the
bearing
(kN) (each bearing has its own coefficient)
bearing
load
coefficient
– Axial bearing
load
coefficient
(each bearing has its own coefficient)
– Radial
Friction Moment
Friction Moment is describing the resistance of the bearing to motion. When a torque equation
of motion mechanism is calculated, these characteristic has to be taken into account. There are
bearing with extremely low friction moment and some with higher friction moment.
The friction moment is calculated using the following formula:
– Friction moment of the bearing (Nmm)
- Coefficient of friction of the bearing (see table)
– Equivalent dynamic load (N)
– Inner diameter of the bearing (mm).
Friction Coefficient
0.0015
0.0020
0.0024
0.0024
0.0010
0.0011
Type of Bearing
Deep groove ball bearings
Angular contact ball bearings single row
Angular contact ball bearings double row
Angular contact ball bearings four-point contact ball bearings
Self-aligning ball bearings
Cylindrical roller bearings with cage, when Fa≈0
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Bearing Selection - How To Select A Bearing?
0.0020
0.0022
0.0018
0.0018
0.0016
0.0013
0.0050
0.0050
0.0018
Cylindrical roller bearings full complement, when Fa≈0
Needle roller bearings
Tapered roller bearings
Spherical roller bearings
CARB toroidal roller bearings
Thrust ball bearings
Cylindrical roller thrust bearings
Needle roller thrust bearings
Spherical roller thrust bearings
A video demonstrating a production process of bearings: How To Select A Bearing? - Bearing Types Ball Bearing
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Bearing Selection - How To Select A Bearing?
By Silberwolf [ CC-BY-2.5 ], via Wikimedia Commons
Rolling bearing based on spherical bearing bodies. Designed for higher speeds and smaller
loads due to small contact area between the balls and the rings. Can also carry axial load in
addition to radial loads. Cheap bearings relative to other types of bearings. Angular Contact Ball Bearing
© SKF
Bearing is based on the spherical bearing bodies. The difference between this type of bearing
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Bearing Selection - How To Select A Bearing?
and the previous one is that the internal slide rails are asymmetric. These bearing support much
better combined loads. It is important to diagnose the loading direction. Positioning the bearing
in the wrong loading direction can cause malfunction in the robot or the moving mechanical
mechanism.
Self Aligning Ball Roller Bearing
This bearing is very similar to the rolling ball bearing except that the slide rails are spherical
instead of straight ones. This means that when a mismatch is created between the tracks of the
inner ring and the outer ring tracks, the rolling balls can align itself in the center of the sliding
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Bearing Selection - How To Select A Bearing?
tracks automatically. Ball Thrust Bearing
Thrust bearing based on spherical bearing bodies. Unlike the standard ball bearing, these
bearing designed to withstand much higher axial forces and not intended to deal with radial
force.
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Bearing Selection - How To Select A Bearing?
By Silberwolf [ CC-BY-2.5 ], via Wikimedia Commons
Cylindrical Bearing
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Bearing Selection - How To Select A Bearing?
Rolling bearing based on cylindrical bearing bodies. Designed for lower speeds applications due
to increased friction but can deal with high loads and forces. Bearing performance lowered
drastically if the sliding tracks are not aligned with each other.
By Silberwolf [ CC-BY-2.5 ], via Wikimedia Commons
Cylindrical Thrust Bearing
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Bearing Selection - How To Select A Bearing?
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Silberwolf
Needle Bearing
Rolling bearing based on needles as the body bearing. The needles are actually long cylinders
with very small diameter. Due to the small diameter of the rolling bodies (the needles), the inner
ring and the outer ring are very close one to the other. This contributes to a design when one
has to choose an internal diameter bearing closely as possible to the axis. Bearing is
characterized by quick degradation and low reliability.
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Bearing Selection - How To Select A Bearing?
Tapered Bearing
Rolling bearing based on conical bearings bodies. The cones roll on the inner rail that is shaped
as conical rails too. Most of the bearing can deal with axial and radial forces but this type of
bearing can deal much better with combined and inclined loads. Usually, they are more
expansive due to the complication of the manufacturing process.
By Silberwolf [ CC-BY-2.5 ], via Wikimedia Commons
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Bearing Selection - How To Select A Bearing?
Tapered Thrust Bearing
Thrust bearing based on conical bearings bodies. Bearing that, unlike standard tapered bearing,
can deal with much higher axial loads and almost no radial loads. These bearings are usually
expensive due to the complexity in manufacturing.
By Silberwolf [ CC-BY-2.5 ], via Wikimedia Commons
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Bearing Selection - How To Select A Bearing?
Spherical Roller Bearing
Rolling bearing based on barrel-shaped bearing body (thick center and thinner at the edges).
This bearing can deal much better with non-concentric bearing rings. Bearings are expensive
due to their manufacturing complexity.
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Bearing Selection - How To Select A Bearing?
By Silberwolf [ CC-BY-2.5 ], via Wikimedia Commons
Fluid Dynamic Bearing
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Bearing Selection - How To Select A Bearing?
These bearings consist of two sealed rings and no internal bearing elements. Pressurized liquid
(usually, oil, water or air) is put inside and between the two rings. The advantages of these
bearings are low cost and much lower friction mechanical bearings. However, these bearings
are very sensitive to temperature changes and can fail in an instant when subjected to sudden
high loads (shocks).
Spherical Bearing
By Androstachys [ CC-BY-SA-3.0 or GFDL ], via Wikimedia Commons
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Bearing Selection - How To Select A Bearing?
Spherical bearing allows a slight angular movement of the drive shaft relatively to the bearing
rings. The inner ring supports the drive shaft and can turn relatively to the outer ring.
Magnetic Bearing
© NASA
Magnetic bearing is bearing with no internal bearing elements. Bearing supports the rotating
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Bearing Selection - How To Select A Bearing?
axis inside by electromagnetic force that cause “hovering” of the two rings in a constant
distance. In this type of bearing there is no mechanical contact between the two rings. This
bearing has almost zero degradation over time and characterized with extremely low friction.
Another advantage is that this bearing has no speed limit. These bearings can turn around at
any speed. Downside is that these magnets require constant electrical input voltage (for
example, a machine that moves the axis and suddenly there is an electrical failure of the
bearing may be damaged severely due to the collapse of the bearing).
Plain Bearing
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Bearing Selection - How To Select A Bearing?
Plain bearing is the simplest type of bearing without any internal bearing elements and based
on sliding motion. This bearing is built from one ring (sleeve), where the drive shaft is turning
inside it. Sliding bearing is very common and most affordable. It is a good choice where high
radial loads may be subjected but has a very high degradation rate. This type of bearing is not
built for high speed applications due to the high friction
By Silberwolf [ CC-BY-SA-2.5 ], via Wikimedia Commons
How To Select A Bearing - So How To Select A Bearing?
Loads
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Bearing Selection - How To Select A Bearing?
Is there an axial load on the bearing? Is there radial loads? Or maybe a combination of both?
it is important to define the size and the direction of the loads.
Now begins the stage of trial and error. first choose a bearing from the catalog that corresponds
to the expected loads.
Using
the Characteristics of the bearing (
,,
,,
,) Calculate the equivalent dynamic load and the equivalent static load.
First calculate the equivalent static load using the formula:
Compare the equivalent static load to the static safety factor. Choose a safety factor from the
top table and use it in the following formula:
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Bearing Selection - How To Select A Bearing?
Now
check
the bearingone.
manufactures
tables
and
lookselected,
for a bearing
static loaddynamic
factor fits to
the calculated
After a bearing
has
been
checkwith
theaequivalent
load of the bearing. To do so use the following equation:
or
This is the equivalent dynamic load calculated according to the formula:
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Bearing Selection - How To Select A Bearing?
Coefficient depends on the bearing type. m=3 for ball bearings, m=3.3 for sleeve bearings.
This figure describes the bearing life in units of millions of turns.
This figure describes the bearing life in units of millions of hours
– Rotation speed of the bearing
Now estimate how long the bearing should work. If you don’t know exactly how long the bearing
should work, you can use the following table: :
Type of application
300 - 3000
3000 - 8000
8000 - 12000
10000 - 25000
20000 - 30000
40000 - 50000
60000 - 100000
<100000
Machines used in domestic, agricultural equipment, instruments, technica
Machinery intended for short use: Manual appliances, freight elevators in ga
Machines designed to work reliably during a short actions: lifts, cranes
Machines designed to work eight hours a day but not regularly: transmission
Machines designed to work eight hours a day and regularly: metalworking m
Machines for continuous 24 hours a day: Turn in machining equipment, med
Water industry machinery, cable manufacturing machines, equipment, vehic
Large electric machines, power plants, pumps Mines, Mine Fans
Put all the gathered data in the formula to get C. now check the bearing tables for the most
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Bearing Selection - How To Select A Bearing?
suitable
load and
the equivalent
equivalent static
dynamic
load .
If one of the calculated figures is smaller than the one in the table, a “stronger” bearing has to
be selected and the dynamic load has to be recalculated (X and Y are variables and depended
on the selected bearing).
If the two calculated figures are higher than the one appearing in the table the one of the two
following choices can be made:
- Stay with the selected bearing with the knowledge that it has higher requirements than
needed.
- Choose a "weaker" bearing to lower cost and the safetey factor as well.
What is the bearing’s expected angular velocity?
Using bearings manufacturer’s catalogs, you can select the appropriate bearing by the indicated
speed in the technical datasheets.
What are the geometric dimensions of the bearing? 35 / 37
Bearing Selection - How To Select A Bearing?
The critical part is the drive shaft. Therefore, the bearing selection process should be aimed
towards matching the inner diameter of the bearing to the outer diameter of the drive shaft.
What are the environmental conditions of the bearing?
If dust or dirt is expected in the work environment, protected and sealed bearing should be
considered.
If large environment temperature variations are expected, then avoid using hydrostatic bearings.
If a sensitive instruments for EMC or electrostatic positioned near the bearings, then avoid using
magnetic bearings or find a way to protect the sensitive instruments.
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Bearing Selection - How To Select A Bearing?
Written by Eran Cenciper (Robot-and-Machines-Design webmaster)
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